The present invention relates to a liquid level and other height measurements, more specifically, to an electromagnetic method and apparatus for measuring liquid level for different liquids (conducting and non-conducting) and also for measuring clearance or thickness.
The usefulness of the RF or microwave field application for monitoring of liquid level is recognized by the prior art. Such devices can operate with either RF or microwave excitation. When an electromagnetic field is excited in the container partially filed with liquid, parameters of the electromagnetic field, such as resonant frequency, vary with the level of the liquid. In particular the state of the art is shown in V. A. Viktorov xe2x80x9cMicrowave Method of Level Measurementxe2x80x9d, The Resonance Method of the Level Measurement, Moscow: Energija. 1987, disclosing an electrodynamic element, made as section of a long line, inserted into a monitored container where the resonant frequency is measured.
A general discussion, see Viktorov V. A., Lunkin B. V., Sovlukov A. S. xe2x80x9cMethod of and Apparatus for Level Measurement by Hybrid Electromagnetic Oscillation Excitationxe2x80x9d, Radio-Wave Measurements, Moscow: Energoatomizdat, 1989, states that an electrodynamic element is placed in a monitored container, and the element""s resonance frequency has a direct correlation to the level of the liquid within the container.
Slowed electromagnetic waves and slow-wave structures are also well known in the field of microwave engineering, see J. R. Pierce, xe2x80x9cTraveling-Wave Tubesxe2x80x9d D. Van Nostrand Company, Inc., Princeton, N.J., 1950. These waves are electromagnetic waves propagating in one direction with a phase velocity xcexdp that is smaller than the light velocity c in a vacuum. The relation c/xcexdp is named slowing or deceleration and is designated as n. In the most practically interesting cases, slowed electromagnetic waves are formed in slow-wave structures by coiling one or two conductors (for example, into a helix, as it is shown in FIG. 1 (prior art), where the other conductor is a cylinder), which increases the path length traveled by the wave, or by successively connecting resonant elements or cells, energy exchange between which delays the phase of the wave, or by using an electrodynamically dense medium (usually a dielectric), or a combination of these methods. Additional deceleration was also obtained due to positive electric and magnetic coupling in coupled slow-wave structures, see V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, # 12, 1995, pp. 1369-1375.
The slow-wave structure-based sensitive elements are known in the art, see Yu. N. Pchelnikov, I. A. Uvarov and S. I. Ryabtsev, xe2x80x9cInstrument for detecting Bubbles in a Flowing Liquidxe2x80x9d, Measurement Techniqies, Vol. 22, # 5, 1995, pp.559-560, and Yu N. Pchelnikov, xe2x80x9cPossibility of Using a Cylindrical Helix to Monitor the Continuity of Mediaxe2x80x9d, Measurement Techniques, Vol. 38, #10, 1995, pp.1182-1184. The slowing of the electromagnetic wave leads to a reduction in the resonant dimensions of the sensitive elements and this enables one, by using the advantages of electrodynamic structures, to operate at relatively low frequencies, which are more convenient for generation and are more convenient for primary conversion of the information signal, but sufficiently large to provide high accuracy and high speed of response. The low electromagnetic losses at relatively low frequencies (a few to tens of megahertz) also helps to increase the accuracy and sensitivity of the measurements. The slowing of the electromagnetic wave leads also to energy concentration in the transverse and longitudinal directions, that results in an increase in sensitivity, proportional to the slowing down factor n. See V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, # 12, 1995, pp. 1369-1375.
Most slow-wave structures were made as two-conductor periodic transmission lines (see Dean A. Watkins xe2x80x9cTopics in Electromagnetic Theoryxe2x80x9d, John Willy and Sons, Inc. Publishers). A version is possible when a slow-wave structure contains three or more different conductors. In all cases the slowed wave is excited in the electrodynamic element between different combinations of the two conductors. The coiled conductors increasing the wave path are named xe2x80x9cimpedance conductorsxe2x80x9d, and conductors with simple configuration such as rods, tapes, etc., stretched along the wave propagation direction are named xe2x80x9cscreen conductorsxe2x80x9d, see V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, # 12, 1995, pp. 1369-1375.
Both the prior art and the present invention measure one or more parameters of an electromagnetic field. Some of the prior art methods and present invention use an electrodynamic element, some are made as a resonant cavity filled with measured liquid or made as an electrodynamic element placed in or outside a container. The electrodynamic element is connected to an external RF or microwave signal generator which is used to excite an electromagnetic field. The change in, for example, the level of the liquid, causes a shift in the characteristics of the electromagnetic field in the electrodynamic element. The shift in characteristics correlates to a change, for example, in the level of the liquid within the measured container.
Devices used in the prior art exhibit several problems overcome by the present invention. Previous methods depend upon the sensitivity of a measured parameter of an electromagnetic field to measure level displacement and provide signal resolution. Sensitivity and resolution increase with frequency. However, the increase in frequency is accompanied by an increase in electromagnetic losses, such losses causing a loss of accuracy of the measurement. Besides, it is known that the higher frequency is, the higher is the cost of an electronics. The relatively low accuracy realized from the prior art is also due to resonant frequency dependence on the monitored liquid""s electric parameters. Thus, there is a need in the art for an electromagnetic method and apparatus for monitoring liquid levels and other heights measurements that has better sensitivity, better resolution, greater diversity and lower cost.
The present invention employs slow-wave structures in electrodynamic elements Contrary to the capacitance and inductance sensitive elements, slow-wave structure-based sensitive elements are electrodynamic elements and can be characterized by the electrodynamic parameters such as resonant frequencies, Q-factor or attenuation, phase shift, etc. The main advantages of such xe2x80x9csensitive electrodynamic elementsxe2x80x9d, in comparison to known ones, are: concentration of electromagnetic energy in a small volume, the independence of their electrodynamic parameters upon the electronic circuit parameters and the dependence on the monitored liquid level or the measured height.
Frequency decrease is achieved due to slowing. Sensitivity increase is achieved due to electromagnetic energy concentration near the surface of the level to be measured and due to splitting electric and magnetic fields for the monitored volume. The measured parameters range is widened due to wide frequency band of slow-wave structures. The application convenience is due to possibility of placing of the electrodynamic element outside the monitoring level. The slow-wave structure-based electrodynamic elements are designed, as a rule, on dielectric base, stable to temperature alteration and its resonant frequency dependence on temperature is very small, contrary to, for example, cavity resonators.
The present invention teaches an electromagnetic method of measuring the liquid level and other heights or other measurements that require high resolution wherein an excited electromagnetic wave with a preset distribution of the electric and magnetic components of the electromagnetic field makes it possible to increase the sensitivity and accuracy of measurement of the level, using relatively low frequencies. The method is implemented in an apparatus, for example, for measuring liquid level, wherein the structural form of the electrodynamic element, used as the sensing element allows increased sensitivity and accuracy. In the invention an electrodynamic element is made at least one section of a slow-wave structure.
It is known, that the dielectric or conducting materials, placed in the electromagnetic field, alter its parameters, for example, its velocity, that leads to the phase delay or resonant frequency alteration. The degree of such alteration and, therefore, sensitivity S is proportional to the relation of the volume V of a material to the monitored volume V0, for example, a volume of a resonator, and depends on the electric and magnetic field distribution in the monitored volume
Sxcx9c(V/V0)F(e,m,s)ƒ, 
where here xcex5 and xcexc are relative permittivity and permeability, "sgr" is conductivity of a material, F(xcex5, xcexc, "sgr") is some function, depending on the material position in the monitored volume V, and ƒ is frequency and the sign means proportionality. See V. A. Viktorov, B. V. Lunkin and A. S. Sovlukov, xe2x80x9cRadio-Wave measurementsxe2x80x9d Moscow: Energoatomizdat, 1989, p. 27. If, for example, dielectric material is monitored, it should be placed in the electric field and its effect will be proportional to the electric field energy in the material. Since the resonant volume V0 is smaller when the first resonant frequency ƒ1 is higher, the sensitivity S rises with frequency increasing. Slowing of an electromagnetic wave n times leads to an n times decrease of the resonant volume V0, that is accomplished by the sensitivity n-times increasing
Sxcx9c(V/V0)nF(xcex5,xcexc,"sgr")ƒ1. 
The sensitivity increasing permits lower frequency and works with smaller losses, which, for example, in conductors are proportional to the square root of frequency. See: E. C. Young xe2x80x9cThe Penguin Dictionary of Electronicsxe2x80x9d, second edition, Penguin Books, p. 530. The electromagnetic losses decrease leads to resolution increase.
When resonant frequency of a metal container, filed with liquid, is measured, the resonant frequency depends not on the liquids level only; it depends on temperature also since the liquid""s permittivity and the container""s volume change with temperature change. The slow-wave structure-based electrodynamic elements are designed, as a rule, on dielectric base and its resonant frequency depends on temperature very small.